1. Field of the Invention
The present invention relates to a lens. More particularly, the present invention relates to a fixed-focus lens.
2. Description of Related Art
Referring to FIG. 1, U.S. Pat. No. 6,989,946 has disclosed a conventional fixed-focus lens 100 applied in a rear projection television (RPTV), which includes a first lens group 110 and a second lens group 120 arranged from an object side to an image side in sequence. A liquid crystal light valve 50 is disposed at the image side. The first lens group 110 includes two lenses 112 and 114 arranged from the object side to the liquid crystal light valve 50 in sequence. The second lens group 120 includes seven lenses 122a, 122b, 124, 126a, 126b, 126c, and 128 arranged from the object side to the liquid crystal light valve 50 in sequence. The lenses 122a and 122b constitute a cemented lens 122, and the lenses 126a, 126b and 126c constitute another cemented lens 126.
Since the field of view (FOV) of the conventional fixed-focus lens 100 is not large enough (the maximum FOV is about 95°), the optical distance between the lens 112 and the screen (not shown) disposed at the object side must be very long for projecting the picture of an appropriate size onto the screen, which results in a large volume of the RPTV. However, if the refractive power of each lens of the fixed-focus lens 100 is adjusted to increase the FOV so as to reduce the above optical distance, the aberration of the picture projected on the screen will be too large.